MG 151 cannon

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The pre-war German doctrine for arming single-engine fighter aircraft mirrored that of the French. This doctrine favored a powerful autocannon mounted between the cylinder blocks of a V engine and firing through the propeller hub, known as a moteur-canon in French (from its first use with the Hispano-Suiza HS.8C engine in World War I, on the SPAD S.XII) and by the cognate Motorkanone in German by the 1930s. The weapon preferred by the French in this role was the most powerful 20mm Oerlikon of the time, namely the FFS model, but this proved too big for German engines. Mauser was tasked to develop a gun that would fit, with a minimum sacrifice in performance. (As a stop-gap measure, the MG FF cannon was developed and put in widespread use, but its performance was lackluster.)[3]

Production of the MG 151 in its original 15 mm calibre format began in 1940. After combat evaluation of the 15 mm cartridge as the main armament of early Messerschmitt Bf 109F-2 fighters, the cannon was redesigned as the 20 mm MG 151/20 in 1941 to fire a 20 mm cartridge. Combat experience showed that a more powerful explosive shell was preferable to a higher projectile velocity.[2] The MG 151/20 cartridge was created by expanding the neck of the cartridge to hold the larger explosive shell used in the MG FF cannon, and shortening the length of the cartridge case holding the longer 20 mm shell to match the overall length of the original 15 mm cartridge.[2] These measures simplified conversion of the 15 mm to the 20 mm MG 151/20 simply by changing the barrel and making other small modifications. A disadvantage of the simplified conversion was reduction of projectile muzzle velocity from 850 metres per second (2,800 ft/s) for the 15 mm shell to 700 metres per second (2,300 ft/s) for the larger and heavier 20 mm shell.[1] With an AP projectile the new 20mm cartridge could only penetrate around 10-12mm of armor at 300m and at 60 degrees, compared to 18mm penetration for its 15mm predecessor in the same conditions, but this was not seen as a significant limitation.[2] The 20 mm version thus became the standard inboard cannon for the Bf 109F-4 series onwards.[2] The 20 mm MG 151/20 offered more predictable trajectory, longer range and higher impact velocity than the 580 metres per second (1,900 ft/s) cartridge of the earlier MG FF cannon.[1] The MG FF was retained for flexible, wing and upward firing Schräge Musik mounts to the end of the war.[2]

The German preference for explosion rather than armor penetration was taken further with the development of the Minengeschoß ammunition, first introduced for the MG FF (in the Bf 109 E-4), and later introduced for the MG 151/20 as well. Even this improvement in explosive power turned out to be unsatisfactory against the four-engine bombers that German fighters were up against in the second part of the war. By German calculations, it took about 15-20 hits with the MG 151/20 ordnance to down a heavy bomber, but this was reduced to just 3-4 hits for a 30 mm shell, from the shattering effects of the hexogen explosive in the shells used for both the long-barreled MK 103 and shorter barreled MK 108 cannon. (Only 4-5 hits with 20 mm calibre ordnance were needed for frontal attacks, even on B-17s, but such attacks were difficult to pull off.) The 30 mm MK 108 cannon thus replaced the MG 151/20 as the standard, engine-mount Motorkanone center-line armament starting with the Bf 109 K-4, and was also retrofitted to some of the G-series.[4]

After World War 2, numbers of ex-Luftwaffe MG 151/20 cannon were removed from inventory and from scrapped aircraft and used by various nations in their own aircraft. The French Air Force and French Army aviation arm (ALAT) utilized MG 151/20 cannon as both fixed and flexible armament in various aircraft, including helicopters. The FAF and ALAT jointly developed a rubber-insulated flexible mount for the MG 151/20 for use as a door gun, which was later used in combat in Algeria aboard several FAF/ALAT H-21C assault transport helicopters and on HSS-1 Pirate gunship helicopters. French Matra MG 151 20mm cannons were used by Portugal and Rhodesia[6] fitted to their Alouette III helicopters, while Denel designed its own variant for the South African Air Force.[7]

Recent developments of 14.5mm High Explosive Incendiary[citation needed] rounds may be regarded as a revival of the 15mm cannon concept.

Two versions of the 20 mm MG 151 were built. Early guns used a percussion priming system, and later E-models used electrical priming. Some rounds were available with a timer self-destruct and/or tracer (or glowtracer). There were also different types of high explosive shell fillings with either standard PETN, a mixture called HA41 (RDX and aluminium), and a compressed version where more explosives were compressed into same space using large pressures (XM).

A little known US research program, called T17, reverse engineered an MG151 and adapted it for the US .60 caliber round, previously intended for an anti-tank rifle. Around 300 of these T17 guns were built, but none saw service, despite the availability of 6 million rounds of .60 caliber ammunition.[9] Almost one million rounds were fired during the T17 testing program. The main US version produced, the T17E3, was made by Frigidaire. Further refinements led to the T39 and T51 versions, but these also did not enter service.[10]

^Anthony G. Williams (2002). Rapid Fire: The Development of Automatic Cannon, Heavy Machine-Guns and Their Ammunition for Armies, Navies and Air Forces. Airlife. p. 154. ISBN978-1-84037-435-3.

^George Chinn 1951, The Machine Gun: Development During World War II and Korean Conflict by the United States and their Allies of Full Automatic Machine Gun Systems and High Rate of Fire Power Drive Cannon, Volume III, Parts VIII and IX. pp. 110-152